The effective extension of the communication distance and the data capacity in the next-generation optical signal technology is no longer a theoretical possibility, becoming the feasible solutions for the modern optical network. This is due, mainly, to the combination of several critical technological advancements, such as, 1) coherent optical detection development, 2) QAM format adoption, 3) progress in adaptive electrical equalization technology.
A typical single-polarization transmitter configuration suitable for generating arbitrary PSK/DPSK signals, which has recently been employed with high-order phase modulation, uses binary electrical driving signals and is composed of a combination of an optical IQ modulator and consecutive phase modulators, as depicted in FIG. 1. The optical IQ modulator, whose Mach-Zehnder modulators (MZMs) are driven at the minimum transmission point, accomplishes a quaternary phase modulation, and higher-order phase modulation signals are generated by the consecutive phase-modulators (PMs). Such PSK/DPSK transmitters are extended by an additional intensity MZM at the end, see FIG. 1, for a Star-QAM generation with symbols placing at the different intensity levels.
The high-speed wide-bandwidth Lithium Niobate (LN) modulators are typically based on either an X-cut (Y-propagating) or Z-cut MZM with traveling-wave electrodes. The single-mode optical waveguides can be fabricated by either Ti-in-diffusion or annealed proton exchange. In a single MZM the refractive index is changed by an externally applied voltage. Such transmitters can be enhanced to polarization multiplexing using polarization-rotators converters that can be built-in into the same LN chip.
On the receiving side a coherent detection is implemented to decode the received optical beam. In contrast to existing optical direct-detection system technology, an optical coherent detection scheme would detect an optical signal's amplitude as well as its phase and polarization. Within the fixed optical bandwidth more data can be transmitted using a coherent detection scheme with increased detection capability and spectral efficiency. Coherent detection provides increased receiver sensitivity by 2-6 dB compared to an incoherent system. In addition, since coherent detection enables an optical signal's phase and polarization to be measured and processed, the transmission impairments that previously presented challenges to accurate data reception can, in principle, be mitigated electronically when an optical signal is converted into the electronic domain. Tier-1 network providers have now realized the potential for optical coherent systems by deploying DPSK systems with improved DSP receiving circuits based on complicated optical phase-lock loops.
FIG. 2 shows the basic setup of a digital coherent receiver with homodyne synchronous detection and polarization division de-multiplexing.
The signal launched into the receiver is split by a polarization beam splitter (PBS). Than both polarization components are interfered with the LO light in two 2×4 90° hybrids. In practice, both separated polarization components of the information signal at the PBS outputs exhibit the same linear polarization state, and it suffices when the LO light, whose polarization must then be aligned to the polarization of the signal at the two PBS outputs, is equally split with a 3 dB coupler. The carrier synchronization is performed by digital signal processing (DSP). The output signals of the two 2×4 90° hybrids are detected by two pairs of balanced detectors which provide the in-phase and quadrature photocurrents of both polarization components at the outputs of the optical receiver frontend. In the electrical receiver part, the in-phase and quadrature signals are sampled by A/D-converters and then further processed by elaborate digital signal processing.
Arbitrary formats modulation formats can be demodulated by such a receiver since demodulation is based on absolute phases. For the detection of any modulation format, the same optical frontend can be used. The digital algorithms and the data recovery must be adapted in accordance with the particular received format. Receiver sensitivity is also increased in comparison with receivers based on differential detection. Moreover, the availability of the optical phase information in the electrical domain enables an efficient digital equalization to compensate for transmission impairments.
The optical hybrids show in FIG. 2 are the critical part of the coherent receiver needed to combine a local oscillator wave, Lo, with the received signal, S. Such an optical hybrid is a key component in phase- or polarization-diversity schemes. Ideally, the hybrid should combine waves with quadrature relative phases at the outputs, providing the advantage of base-band processing. In a two-phase case outputs must be mutually phased at 90° (in-phase and quadrature, referred to as I and Q signals).
It would be desirable to create highly integrated devices with functionality shown in FIG. 1 and FIG. 2, preferably built-in within the same electro-optical chip, sharing a single package with electronics components.
The goal of this invention is to use coherent transmitters/receivers, and channel compensation algorithms to achieve agile free-space optical communications links. The flexible, highly integrated architecture will provide a secure, robust, multi-rate/multi-format cost-efficient optical transmission that is resistant to jamming and eavesdropping and achieves spectrally-efficient high data rate throughput in any challenging communication environment.